1. Field of the Invention
The present invention relates generally to a method of manufacturing a double layer resist pattern in the course of manufacturing semiconductor devices and the like and, more specifically, to a manufacturing method improved to provide highly precise double layer resist pattern having higher resistance against dry etching. The present invention further relates to a double layer resist structure providing such double layer resist patterns.
2. Description of the Background Art
As the degree of integration of a semiconductor device has increased recently, a technique capable of forming a fine resist pattern having a high aspect ratio in a manufacturing process has been increasingly demanded. Thus, the technique of forming a pattern using double layer resist has been proposed as a technique satisfying the demand. As this kind of method, for example, in SPIE Vol. 174, 114(1979), a first conventional method for forming a pattern using double-layer resist, comprising polymethylmethacrylate (hereinafter referred to as a PMMA) on a lower layer and a positive photoresist on an upper layer is proposed. FIGS. 6A to 6E are sectional views of a pattern during its manufacturing processes. Referring to these figures, a description is made of a conventional method for forming a fine pattern.
Referring to FIG. 6A, a PMMA photoresist is applied onto a substrate 1 to form a PMMA photoresist layer 2 serving as a resist of the lower layer. Then, a g-line photoresist photosensitive to a g-line having a wavelength of 436 nm is applied onto this PMMA photoresist layer 2 to form a g-line photoresist layer 3 serving as a resist on the upper layer. Thus, the double-layer resist is formed on the substrate 1.
Referring to FIG. 6B, exposure to a g-line 5 is performed using a mask 4 having a predetermined pattern. Since the PMMA photoresist layer 2 is not exposed to the g-line 5, a predetermined portion 3a (hereinafter referred to as a g-line exposure portion 3a) in the g-line photoresist layer 3 is only photosensitized.
Referring to FIG. 6C, when development is performed by alkali solution, only the g-line exposure portion 3a is only removed and a portion 3b unexposed to the g-line (hereinafter referred to as a g-line un-exposure portion 3b) remains on the PMMA photoresist layer 2.
Referring to FIG. 6D, the whole surface of the substrate 1 is exposed to a deep UV ray 8. In this case, since the g-line un-exposure portion 3b intercepts the deep UV ray 8, this g-line un-exposure portion 3b serves as a mask. As a result, a portion 2a of the PMMA photoresist layer 2 (hereinafter referred to as a deep UV-ray exposure portion 2a) exposed to the deep UV ray 8 is only photosensitized.
Referring to FIG. 6E, when development is performed by organic solvent, the deep UV ray exposure portion 2a is only selectively removed and a portion 2b unexposed to the deep UV ray 8 (hereinafter referred to as a deep UV ray un-exposure portion 2b) remains on the surface, so that a double-layer resist pattern 6 comprising the deep UV ray un-exposure portion 2b and the g-line un-exposure portion 3b is formed on the substrate 1.
Then, etching is performed on an aluminum wiring film and the like serving as a material to be etched and formed on the substrate, using this double-layer resist pattern 6 as a mask.
A first conventional method for forming a pattern is thus structured and it is characterized by the use of the PMMA photoresist layer 2 as a photoresist of the lower layer as shown in FIG. 6D. However, there was a disadvantage that it takes long to photosensitize the PMMA photoresist layer 2 and throughput (the number of wafers to be processed per line time) is low because the PMMA photoresist has low sensitivity. In addition, as shown in FIG. 6E, there was another disadvantage that it is impossible to form a highly precise and fine pattern in a material to be etched when the pattern 6 of the double-layer resist is used as a mask in a subsequent etching process because the PMMA photoresist layer 2 is inferior in dry etching resistance.
The double layer resist method employing PMMA as a photoresist of the lower layer, an alkali developer is used for developing the upper layer resist, while Methyl Isobuthyl Ketone which is an organic solvent, is used for developing the lower layer resist. Referring to FIG. 7 (corresponding to FIG. 6D), when the upper layer resist happens to be left due to insufficient developing or the like, there is a mixing layer 55 generated at the interface between the upper and lower layer resist. Since the mixing layer 55 is insoluble in organic solvent, it prevents the succeeding step of developing the lower layer resist by methyl isobutyl ketone, causing another disadvantage that the lower layer photoresist is not developed satisfactorily.
In a second prior art disclosed in Japanese Patent Laying-Open No. 62-183449, a novolak photoresist having high sensitivity and high resistance against dry etching is employed as the resist of the lower layer. The technique employing the novolak photoresist as the lower layer is also disclosed in SPIE vol. 771, 273 (1987).
FIGS. 8A to 8B show process steps for forming the double layer resist pattern disclosed in Japanese Patent Laying-Open No. 62-183449. The process steps of the second prior art will be described in the following with reference to these figures.
Referring to FIG. 8A, an Al layer 51 which is to be the interconnection metal is formed on a substrate 50.
Referring to FIG. 8B, MP-2400 (novolak photoresist produced by Shipley Company) is applied as the lower layer resist 52.
Thereafter, as shown in FIG. 8C, a photoresist formed of silyl ether of naphthoquinone diazide sulfonate of the novolak resin is formed as the upper layer resist 53. Thereafter, exposure is carried out by an optical stepper employing g-line of a mercury lamp. Referring to FIG. 8D, development is carried out by using a mixture of monochlorobenzene and cyclohexane (2:1 weight proportion) to form the pattern of the upper layer resist 53.
Thereafter, the substrate 50 is entirely irradiated by the light having the wavelength of 200 to 300 nm by using a 500 W Xe-Hg lamp.
Referring to FIG. 8E, development is carried out by using a solution of MP-2401 developer (developer for MP-2400) attenuated to 5 times by water for 30 sec. The double layer resist pattern 54 is formed through these steps.
Thereafter, the Al layer 51 is dry etched by a conventional method using the double layer resist pattern 54 as a mask, the double layer resist pattern 54 is removed, and the Al interconnection pattern shown in FIG. 8F is provided.
The above described second prior art has the following disadvantages.
Referring to FIG. 8D, the light possibly be transmitted through the upper layer resist 53 as the upper layer resist 53 does not fully function as the mask when the substrate 50 is irradiated by light. Consequently, referring to FIG. 9A, the thickness of the double layer resist 54 is reduced. When the Al layer 54 is dry etched by using the thin double layer resist 54, the dry etching process of the Al layer 51 does not proceed as expected due to the insufficient thickness of the double layer resist 54, and the desired pattern of the Al interconnection can not be provided, as shown in FIG. 9B.